Best I can find so far is this, which seems very similar to the method described, but is quite a bit later (1982): https://patentimages.storage.googleapis.com/

Best I can find so far is this, which seems very similar to the method described, but is quite a bit later (1982): https://patentimages.storage.googleapis.com/3d/d5/b4/5beda2898d9cda/US4644422.pdf https://patentimages.storage.googleapis.com/3d/d5/b4/5beda28... > An anti-copy system which prevents copying of tapes and records and other source material by placing a degrade signal on an analog source signal to be recorded on a recording device. One embodiment of the system includes a pulse width modulation (PWM) circuit that generates a pulsed signal in a frequency range from near the upper end to well above the range of hearing. > However, attempts to make taped copies of the original recordings cause the degrade signal to beat against the bias oscillator frequency of the tape recording head, thereby creating harmonic signals that result in distortion and noticeable alteration of the reproduced source signal that are unpleasant and objectionable to the listener.